Long dash interrupter



Oct. 19, 1943. E. HAJos LONG DASH INTERRUPTER Filed Jan. 4, 1941 2Sheets-Sheet l r @III Ybbww ma,

ATTORNEY Patented Oct. 19, 1943 LONG DASH INTERRUPTER Eugene Hajos,Jamaica Estates, Long Island, N. Y., assignor to Radio Corportion ofAmerica, a corporation of Delaware Application January 4, 1941, SerialNo. 373,071

5 Claims.

The present invention concerns an arrangement for interrupting unwanted'long dashes accidentally left on transmitter keying circuits.

In the remote keying of radiol transmitters by automatic sendingdevices, trouble is frequently experienced due to'long' dashes leftoncontrol lines. The auto occasionally sticks during transmission, orisy stopped at the termination of transmission-,squarely on a dash.Since one attendant is calledupon; to supervise many circuits, both; attheftranic-center and at the transmitting station, this condition attimes remains` unnoticed fcr'manyv minutes, especially when theattendant is occupied. withl another circuit. Besides.- radiating.unnecessary signals and wasting power, longv dashes: consume,unproductively, a valuable part of the useful lifeV ofthetubes in atransmitter. In addition, service is` interrupted when one or moreytubes. are renderedlowemission by a long.' dash.

Thel foregoing dicuities areovercomc by the present inventionwhichprovides means for opening the local4 keying circuit of the transmitterwhenever. a dash occurs which' exceeds a predetermined: time. limit.This means includes a relay and: a timing element whcsesettingdetermines; theY time of operationof the relay. In accordance with theinvention, the keying circuit is automatically restored-to the-normalyoperating conditionl as soon as the. long dash has been removedfromtheccntrollineby the remote operator.

A better understanding cfthe invention may be hadV by referringtothefollowing description which is accompanied by drawings, wherein Fig. 1illustrates the' preferred circuit arrangement of. the invcntion,.andFigs.,2 and\3'show graphs giving relationswhich a-idf inthe constructionof thecircuit of Fig. 1 for certain desired condi.- tions.

Referring to Fig. l of the drawings, there is shownv schematically' astandard` keying circuit employed in a short wave transmitter which hasbeen modified by the addition of certain elements containedi in box I0.Only those circuit elements have been shown which it is believed arenecessary for an. understanding ofy the principles underlying; theinvention. The keyed transmitter is indicated: only by the keyed radiofrequency stage. I2 whose. output; is suitably coupled through leadgII:to anantenna,.no.t. shown. Transmitter stage; IZiskeyed by a pairoffkeyer vacuum tubes I4',A I4 acting in; parallel, said. keyer tubes intum being controlled by a polarized keying relayF 5;. Itzshouldbenoted;.that the mar contact of relay 5 is connected to the grids of thekeying tubes I4, I4 while the armature of relay 5is connected to anegative potential of approximately' 200 volts. The winding of relay 5is connected by means of a land line or channel 6 with a distant keyingcircuit located at a remote telegraph station. rhe details of thetransmitter represented by stage l2 and the keying unit I4, I4 are wellknown in the art and form no part, per se, of the present invention.

The circuit elements contained in box il) and which go tc make up thesystem of the invention include a vacuum tube 3 in whose output is anordinary telephone relay 4 and in whose input is a timing circuitconstituted by a capacitor I and a potentiometer 2 connected in shunt tothe capacitor. The grid of tube 3 is suitably tapped to a point on thepotentiometer 2 intermediate its ends. One terminal of the timingcircuit I, 2 is grounded while its other terminal is connected to thespace contact of the polarized relay 5. The armature of relay 4 isconnected'to the marking contact of polarized relay 5, While the contactof relay 4 is connected to the grids of keying tubes It, I4. In thisWay, the armature and contact of relay 4' are serially arranged in theconnection extending between the mark contact of relay 5 and the keyingunit.

In the operation of the system of Fig. l. keyed telegraph signalsappearing on line 6 will operate polarized relay 5 to cause its armatureto alternately engage the mark and space contacts. Let us assume thatthemark contact is engaged when a dash is sent over line 6 While thespace contact is engaged when spacing current appears on line S. Thesame 20D volt negative potential on the armature of polarized relay 5which applies negative potential to the grids of keying unit I4, I4 whenthe armature engages the mark contact will, it will be seen, applynegative potenial to the timing circuit I, 2 when the armature engagesthe space Contact. Whenever space current flows in the keying line,capacitor I will be charged to a potential of 200 volts (negativerelative to ground) A portion of this potential appears on the grid ofvacuum tube 3, its value being dependent cn the adjustment of the tap onpotentiometer 2.

During normal keying conditions, capacitor I will be charged to apotential averaging only slightly less than 200 volts. Potentiometer` 2is adjusted to provide a negative potential on the grid cf'tube 3, inexcess of the value required to stop the plate current. Thus, undernormal conditions of keying, no current will flow in the coil of thetelephone relay 4, and the contacts of the latter will therefore beclosed, providing normal continuity for the local keying circuit.

However, when a long dash occurs on the line 6, thus maintaining thearmature of the keying relay 5 on the mark contact, capacitor I willgradually discharge through potentiometer 2. After an interval of time,dependent on the adjustment of the potentiometer, the negative potentialon the grid of tube 3 will drop sufficiently to permit plate current t0flow, thereby actuating relay 4 and thus opening the local keyingcircuit.

When normal keying is resumed on the control line 6, or a space putthereon, the capacitor I will again be charged to the aforementionednegative potential; which stops the plate current through tube 3. Thearmature of relay 4 being thereby released, continuity is again restoredin the local keying circuit between the grids of tubes I4, I4 and themark Contact of relay 5.

The following is a study of the quantitative relations inherent in thecircuit of Fig 2. The voltage across a capacitor shunted by aresistance, at any time t, after removal of the charging source, isgiven by:

Emme-RC (i) where Ec equals voltage after a time t, Eo equals initial(or source) voltage, e equals 2.7183 t equals time, in seconds, afterremoval of the source, R equals resistance shunting the capacitor (inmegohms), and C equals capacitance (in microfarads) Assuming thecharging source to be 200 volts, and adopting suitable values for R andC, such as 2.5 megohms and 4 microfarads, respectively. graph II of Fig.3 shows the relation between the voltage Ec and the time t, as evaluatedby the above formula. A consideration of this graph will show that, evenduring relatively low keying speeds, the voltage Ec cannot fallappreciably below 200 volts. Thus, at ten words per minute, the averageduration of the marking interval is approximately 1/8 of a second.During this interval Ec can drop, at most. only 3 volts; as may be seenfrom graph II. Thus, for the practical purpose in hand, it may beassumed that the voltage Ec remains substantially at 200 volts duringnormal keying conditions.

The action of the interrupterudevice depends also upon the operatingcharacteristics of vacuum tube 3 and that of relay 4. A relay that willgive positive operation 0n 4 milliamperes, direct current, can easily bepurchased in the market. One such relay sold in the industry has a coilresistance of 200 ohms.

Regarding vacuum tube 3, selection may be made from a multitude ofreceiving circuit tubes, although the R. C. A. 56 is thought to havevery suitable operating characteristics. Fig. 2, graph I, shows thelp/Eg characteristic of this tube at a plate voltage of 120. This platevoltage was chosen because it limits the plate current to maximum of 16milliamperes, even though the bias on the grid should drop to zero. Thismaximum current, though ilcwing through the relay coil indenitely, willcause but negligible heating.

Referring to Fig. 2, it is seen that Ip will be 4 milliamperes when thenegative bias on the grid of the tube drops to 5 volts. Although thestatic characteristic has been employed in this determination, it is tobe noted that the direct current plate resistance oi the tube at thispoint of the curve is approximately 30,000 ohms, compared with a relayresistance of 2,000 ohms. The small drop in plate voltage caused by theintroduction of the relay does not affect the operation to anynoticeable extent.

It will now be of interest to study the relationship between thepotentiometer setting and the timing of the circuit. As beforementioned, relay 4 will operate when the plate current reaches a valueof 4 milliamperes, a condition realized the moment the grid potentialdrops to -5 volts. Now the grid potential at any time is given by thefollowing relationship:

Ec T2,500,000 (2) where Ec equals voltage across capacitor I, Eg equalsvoltage on the grid, r equals resistance included between potentiometerslider and ground. (Total resistance across capacitor equals 2,500,000ohms.) Setting Eg equal to 5, and transposing, we now get:

12,500,000 Tn Ec olnr s (3) The procedure for setting the device maybest be considered by means of an example. Suppose it is desired thatthe device interrupt long dashes of ten seconds duration, and no less;then consulting Fig. 3, graph II, we flnd that Ec drops to '74 voltswhen a dash has been on for ten seconds. Substituting this value of Ecin Equation 3 gives us a value of 169,000 ohms for r. This, then, is thepermanent setting of the potentiometer for the interruption often-second dashes.

To facilitate the making of settings, without the necessity for anycalculations by the operator, graph III of Fig. 3 has been given to showat a glance the required setting of the potentiometer, in terms of 1',for any value of time from zero to thirty seconds. While circuitinsulation resistance, and the deviation of individual tubes and relaysfrom their average characterlstics, have not been considered in thepreparation of this curve of Fig. 3, it is nevertheless sufficientlyaccurate for practical use. Circuit insulation re-` sistances of theorder of 15 to 20 megohms are easily realizable, employing ordinary 600volt switchboard Wire and inexpensive condensers. Time settings may alsobe made by the cut-andtry method; a procedure which may be required incases Where the keying bias of a transmitter dilers appreciably from 200volts.

An advantage of the invention lies in the fact that once a suitablevalue of timing has been decided upon, and the potentiometer setaccordingly, the circuit of the invention requires no further attentionon the part of the operator, with the exception of infrequent tuberenewals. Furthermore, the device can cause no interference with thelocal keying of the transmitter by the attendant, during tuningadjustment, as relay 5 ls then out of service.

In one embodiment tried out in practice, a Western Electric 209 FAkeying relay 5 was employed and carefully observed to detect anydeleterious sparking on the space contact, lncident to the operation ofthe timing circuit. During normal keying, absolutely no sparking couldbe detected. This is because, for this condition, the space contact andtongue o! the relay do not diier materially in potential and thereforelittle current is handled by these contacts. After a long dash, however,the capacitor I is in the discharged state and, upon restorationv ofspace current'` on. they line by the remote operator, current flows fromtongue to space contact of the.A relay to recharge the capacitor. On@these occasions a barely perceptible spark sometimes occurred; However,it was always of lesser magnitude than the sparking noticeable on thekeying (or mark) contact of the relay. Since the charging of thecapacitor is practically instantaneous, the space contact is nevercalled upon to break any current; accordingly sparking never occurred onthe break at this contact. No provisions for spark suppression needtherefore be made.

What is claimed is:

l. In combination, a telegraph line, a polarized relay connected to saidline and responsive to marking and spacing currents sent over said line,said relay having an armature and oppositely disposed marking andspacing contacts, a transmitter for sending out marking impulsesinterspersed with spacing intervals, an electron discharge devicecircuit for keying said transmitter, a connection from said dischargedevice circuit to the marking contact of said polarized relay, aconnection from the armature of said relay to a source of potential, acondenser connected to the spacing contact of said relay for storing anelectric charge during spacing intervals only, and means responsive to adiminution of said charge on said condenser below a predetermined valuefor breaking the connection between said electron ischarge devicecircuit and the marking contact of said polarized relay.

2. In combination, a telegraph line, a polarized relay connected to saidline and responsive to marking and spacing currents sent over said line,said relay having an armature and oppositely disposed marking andspacing contacts, a transmitter for sending out marking impulsesinterspersed with spacing intervals, a keyer for said transmitter, a.connection from said keyer to the marking contact of said polarizedrelay, a connection from the armature of said polarized relay to thenegative terminal of a source of unidirectional potential, a timeconstant circuit composed of a condenser connected to the spacingcontact of said polarized relay for storing an electric charge duringspacing intervals only and a resistor in shunt to said condenser, anelectron discharge device having its control electrode connected to apoint on said resistor intermediate the ends thereof, said point beingchosen to provide a negative bias on the control electrode of saiddevice which prevents the ilo-w of current through the device at normalsignaling speeds by virtue of the negative potential supplied to saidtime constant circuit during spacing intervals, each spacing intervalbeing sufcient to charge said time constant circuit to a value whichprevents the flow of current through said last electron dischargedevice, and a relay in the output circuit of said device controlling thecontinuity of the connection between said keyer and the marking contactoi said polarized relay, said last electron discharge device beingresponsive to the dissipation of the electric charge on said condenserbelow a predetermined value caused by the absence of spacing current onsaid telegraph line for a predetermined interval to produce a ilow ofcurrent in said output circuit ior interrupting the connection betweensaid keyer and said polarized relay.

3. In a radio communication system, in combination, a telegraph station,a remote station having a radio transmitter associated therewith,

a linelextending; between said stations, an electromagnetic relayat:said remote station operativelyassociated'with. said. line andresponsive to markingand spacing currents: sent over said line,

f said, relay` having,y an; armature and oppositely disposed marking andspacing contacts, a connection from said armature, to the negativeterminal of a source of potential, a keyer connected to said markingcontact and controlled by said relay for keying said radio transmitterin accordance with the signal characters received over said line, avacuum tube circuit including a vacuum tube having a grid, an anode anda cathode, a time constant arrangement composed of a condenser shuntedby a resistor connected between said cathode and said spacing contact, aconnection from the control electrode of said vacuum tube to saidresistor, and a connection from said anode to a circuit for controllingthe continuity of the connection from said keyer to said relay, eachengagement of said armature with said spacing contact being sufficientto charge said time constant circuit to a value necessary to prevent theflow of current in the anode circuit of said vacuum tube, whereby only acontinuous marking current over said line for a predetermined period oftime Will cause the new of current through said vacuum tube circuit witha resulting interruption between said keyer and relay.

4. In combination, a telegraph line, an electromagnetic keying relayconnected to said line and responsive to marking and spacing currentssent over said line, said relay having an armature and oppositelydisposed marking and spacing contacts, a radio transmitter for sendingout marking impulses interspersed with spacing intervals, an electrondischarge device keyer for said transmitter, a connection from the gridof said keyer device to the marking contact of said keying relay, aconnection from the armature of said keying relay to the negativeterminal of a source of unidirectional potential, a time constantcircuit composed of a condenser and a resistor in shunt to saidcondenser connected to the spacing contact 0i said keying relay forstoring an electric charge during spacing intervals only, a vacuum tubehaving its control electrode connected to a point on said resistor sochosen as to provide a negative bias on the control electrode of saidtube which prevents the ilow oi current through' the tube at normalsignaling speeds by virtue of the negative potential supplied to saidtime constant circuit during spacing intervals, each spacing intervalbeing suilicient to charge said time constant Circuit to a value whichprevents the flow of current through said last tube, and anelectromagnetic relay inA the output circuit of said tube controllingthe continuity of the connection between said keyer and the markingcontact of said keying relay, .said tube being responsive to thedissipation of the electric charge on said condenser below apredetermined value caused by the absence of spacing currents on saidtelegraph line for a predetermined interval to produce a ow of currentin said output circuit for interrupting the connection between saidelectron discharge device keyer and said keying relay.

5. In combination, a telegraph line, a polarized relay connected to saidline and responsive to marking and spacing currents sent over said line,said relay having an armature and oppositely disposed marking andspacing contacts, a transmitter for sending out marking impulsesinterspersed with spacing intervals, a keyer circuit for keying saidtransmitter, a connection from said keyer circuit to the marking contactof said polarized relay, a connection from the armature of said relay toa. source of potential, a condenser connected to the spacing contact ofsaid relay for storing an electric charge during spacing intervals only,and means responsive to a diminution of said charge on said condenserbelow a predetermined value for breaking the connection between saidkeyer circuit and the marking contact of said polarized relay.

EUGENE HAJ OS.

